There has been mounting evidence that estrogens express neuroprotective effects by the suppression of neurotoxic stimuli largely via their direct radical-scavenging activity. The objective of this grant application is to understand this activity by focusing on the underlying chemistry in which the molecular mechanism of the process and the chemical nature and fate of the products derived from the radical-scavenging reaction are considered the key elements. Our hypotheses center on the role of estrogen-derived quinols as specific reaction products whose involvement has been implicated by preliminary data. By systematically varying substituents in the 2- and/or 4-positions of the phenolic A-ring compounds, we should gain insight into the influence of phenoxy-radical (ArO*) stability, electron density of ArO* at the C-1i0 position and, thus, its reactivity towards hydroxyl radical (-0H). The Fenton-reaction, which leads to 0H formation, will be employed as a chemical model. We hypothesize that hydroxylation will be involved upon *0H exposure as an important process that produces non-radical products from estrogens. In the model system that will be used to study the fate of the synthetic estrogens obtained by the systematic modification of the endogenous compounds, only one type of (mono)hydroxylated species, of a quinol structure, is anticipated via a two-step hydroxyl-radical scavenging mechanism. The rate of quinol formation is expected to correlate not only with the steric and electronic elements of the steroidal compounds, but also with their neuroprotective effect. The phenol to quinol pathway may augment the estrogens' free-radical scavenging efficacy, and make a pivotal contribution to neuroprotection. A reductive quinol to phenol transformation (hence, estrogen "recycling") that prevents the depletion of the available neuroprotective estrogens in vivo will also be investigated. This reactivation (in terms of kinetics and influence by the A-ring substituents) will be studied in vitro in cell-free and cellular systems, and by in vivo microdialysis in experimental animals. Support will be sought for the hypothesis that the reduction of quinols is not accompanied by an increased formation of reactive oxygen species. By using glutamate-induced oxidative stress in HT22 cells as an experimental paradigm, we will study the neuroprotective effects of the phenolic A-ring derivatives of estrogens. In addition to structure - activity relationship studies, a correlation between the propensity of the compound to form its quinol product upon -OH exposure and its effective dose in this model system for neuroprotection will be sought. It is anticipated that the new estrogen analogs selected based on mechanistic studies and in vitro screening will show in vivo neuroprotective effects in a transient middle cerebral artery occlusion, a model for cerebral ischemia. Hence, they will serve as new lead compounds for the development of neuroprotective agents with improved efficacy.